1. Field of the Invention
The present invention relates to a communication apparatus, a communication system and a communication control method. More particularly, the present invention relates a spanning tree management bridge for interconnection between networks and to a method of constructing a spanning tree.
2. Description of the Related Art
A spanning tree is used to construct a local area network (LAN). Details of such a spanning tree are specified in IEEE802.1D. In a case where a plurality of LANs are interconnected by bridges, if a loop is formed when a broadcast packet is sent to the LANs, a phenomenon occurs in which passing of the broadcast packet through the loop is endlessly continued and the broadcast pack does not disappear. For the purpose of preventing such a phenomenon, a point in the communication path connected in loop form is logically disconnected by processing under a protocol called a spanning tree protocol (bridge protocol) to form a tree structure.
In a bridged LAN in which a plurality of LANs are interconnected by bridges, there is a need to divide the above-mentioned spanning tree domain for reasons described below. The first reason is because it is necessary to reduce unnecessary topology changes after occurrence of a fault by reducing the spanning tree reconstruction area in order to shorten the restoration time. The second reason is because it is necessary to reduce routes blocked by a spanning tree by reducing the spanning tree domain in order to eliminate a wasted operating band region.
FIG. 8 shows a bridge model in conformity with IEEE802. 1D. Referring to FIG. 8, the bridge model has ports #1 and #2 respectively connected to LAN segments in a spanning tree domain 100, and media access control (MAC) entities 3 and 4 are provided in correspondence with the ports. Each media access control (MAC) entity includes a frame reception section 10 or 61 and a frame transmission section 11 or 60. A configuration bridge protocol data unit (configuration BPDU) received by the frame reception section 10 or 61 is transmitted to a bridge protocol entity 30 in an upper-layer entity 1 through a logic link control (LLC) entity 20 or 40.
The bridge protocol entity 30 executes processing on the basis of the received configuration BPDU under the spanning tree protocol specified in IEEE802.1D. By executing processing under the spanning tree protocol, the bridge protocol entity 30 determines the port role of each of the ports #1 and #2 (root port, designated port, alternate port) and the port state (forwarding, blocking) and stores them in a port state information storage section 50 or 54 in a MAC relay entity 2.
A MAC relay entity 2 controls transfer or discard of data frames received from the MAC entities 3 and 4. The MAC relay entity 2 includes a forwarding processing section 51 which performs data frame transfer control by referring to the port state information storage section 50 and 54 and a filtering data base 52, and a learning processing section 53 which learns data frame transfer control up to the present time and stores learning results one after another in the filtering data base 52 to update the same.
Each of the frame transmission sections 11 and 61 transmits a data frame transferred from the forwarding processing section 51 to the spanning tree domain 100 through the port #1 or #2, receives a configuration BPDU for forming a spanning tree through the LLC entity 20 or 40, and transmits the received configuration BPDU to other bridges through the port #1 or #2.
In the conventional bridge model shown in FIG. 8, configuration BPDUs from the ports are collected in the common single bridge protocol entity 30 in the upper-layer entity 1. In this one bridge protocol entity 30, information about the port roles and the port state of the ports #1 and #2 is determined. Accordingly, all the ports in one bridge participate in one spanning tree domain 100, as shown in FIG. 8, so that the corresponding spanning tree cannot be divided.
If spanning tree division cannot be performed, a problem described below is encountered. When the link between some of the bridges constituting the network is lost due to a fault in the bridges or a fault in the network connected between the bridges, reconstruction of the spanning tree is performed in accordance with a prescription in IEEE802.1D. When this reconstruction is performed, spanning tree configuration information on the spanning tree constructed before (information stored in the port state information storage sections 50 and 54) and the filtering data base 52 for data packet forwarding that has been obtained by learning are entirely initialized and spanning tree reconstruction is newly performed.
A considerably long time, depending on the number of bridges constituting the reconstructed network of the tree, is required for the reconstruction, and data packets to be transferred onto the network are not transferred until the reconstruction is completed. That is, a condition similar to service interruption occurs. If the scale of the bridge network is larger (the number of bridges constituting the network is larger), this condition becomes marked.
As a method for dividing a spanning tree domain, a method of dividing the network by a router or a virtual LAN (VLAN) may be used. Such a method, however, simultaneously divides a broadcast domain. This means that the service for users is restricted for the network administrator's convenience.